An oxymethylene polymer has excellent mechanical and thermal performance. Particularly, an oxymethylene copolymer has better thermal stability and moldability than an oxymethylene homopolymer, and therefore has been widely used as an engineering plastic. There are a great number of documents concerning the method for producing an oxymethylene copolymer. For example, JP H08-325341 A has a description of a method for producing an oxymethylene copolymer (bulk polymerization method), in which 1,3,5-trioxane (hereinafter, referred to simply as “trioxane”) and a comonomer are polymerized at a temperature of 60 to 115° C. in the presence of a polymerization initiator, and the copolymer formed in the polymerization reaction is subjected to crystallization and precipitation.
JP S58-98322 A discloses a continuous method for bulk polymerization of trioxane, comprising subjecting trioxane and a comonomer to bulk polymerization at a temperature of 135 to 300° C. in the presence of a polymerization initiator, characterized in that the monomer and the formed polymer are present in a molten state during the polymerization.
Further, JP H07-90037 A discloses a method in which a polymerization reaction is conducted for a specific period of time using a protonic acid as a polymerization initiator in a cylindrical reactor having a stationary agitating member, and then the polymerization initiator is quickly deactivated, and further the unstable end (hemiacetal end group) is stabilized in the presence of the residual monomers, and the volatile components are removed by evaporation from the reaction mixture, continuously producing an oxymethylene copolymer having a stable end group in a homogeneous phase.
JP 2010-504380 A discloses a heterogeneous polymerization reaction using a cationic polymerization initiator containing a protonic acid and a salt of a protonic acid. JP 2013-28804 A discloses a method for producing an oxymethylene copolymer, in which the oxymethylene copolymer containing residual monomers obtained in a homogeneous molten state is directly fed into a high-temperature hydrolysis mixture containing a protic solvent and subsequently the oxymethylene copolymer is allowed to precipitate, producing a highly stable oxymethylene copolymer.
On the other hand, in International Patent Application Publication No. 2015/002147, the present inventors have disclosed that it has been difficult to produce an oxymethylene copolymer having a high molecular weight by a conventional method for producing an oxymethylene copolymer in which polymerization is conducted while maintaining the molten state from the start to the termination of the polymerization reaction, but, by cationically polymerizing trioxane and a comonomer at a polymerization temperature of from 135 to 300° C. in the presence of at least one salt of a protonic acid having a molecular weight of 1,000 or less, and at least one polymerization initiator selected from the group consisting of protonic acids having a molecular weight of 1,000 or less, protonic acid anhydrides having a molecular weight of 1,000 or less, and protonic acid ester compounds having a molecular weight of 1,000 or less, a polyoxymethylene copolymer having a high molecular weight can be produced in high yield.